To simplify the following discussion, the present invention will first be explained in terms of a frequency standard. Other applications of the invention will then be discussed below. High-speed communication links that operate at modulation frequencies above 1 GHz have become common in telecommunications and other digital communication links. Such systems have created a need for inexpensive frequency standards that can operate outside the standards laboratory. Such a frequency standard must provide a reliable output signal independent of environmental fluctuations such as temperature and magnetic fields.
One class of frequency standard that has the potential for meeting these needs utilizes Coherent-Population-Trapping (CPT) in quantum absorbers. CPT-based frequency standards are described in U.S. Pat. Nos. 6,363,091 and 6,201,821, which are hereby incorporated by reference. Since such frequency standards are known to the art, they will not be described in detail here. For the purposes of the present discussion, it is sufficient to note that in such standards, the output of an electromagnetic source that has two frequency components (CPT-generating frequency components) that are separated by a frequency difference is applied to a quantum absorber. The quantum absorber has at least two low energy states and at least one high energy state that can be reached by transitions from each of the low energy states. One of these two CPT-generating frequency components in the applied electromagnetic field induces transition from one of the low energy states to the high energy state while the other frequency component induces the transition from the other low energy state to the common high energy state. Thus the quantum absorber absorbs the energy from the applied electromagnetic field.
When the frequency difference between the two frequency components is approximately the same as the corresponding frequency difference between two low energy states in the quantum absorber, the quantum absorber can be in a linear superposition of the two low energy states such that the quantum absorber does not interact with the applied electromagnetic field. This phenomenon is called Coherent-Population-Trapping (CPT). The quantum absorber exhibits an absorption minimum (or a transmission maximum) when the frequency difference between the two frequency components is exactly the same as the corresponding frequency difference between two low energy states in the quantum absorber. A suitable detector measures the intensity of the electromagnetic field transmitted through the quantum absorber. A servo loop can be used to adjust the frequency difference of these two frequency components such that the maximum amount of electromagnetic field leaves the quantum absorber. Hence, the frequency difference of these two frequency components is held at a precise value that is related to the difference in energy of the corresponding low energy states of the quantum absorber. If the difference in energy of the low states in the absorber remains constant, the resultant frequency standard will have a very high precision.
In some frequency standards, a modulated laser is used to produce the CPT-generating frequency components. One or more sidebands from the modulation can be used as the CPT-generating frequency components. In this case, the servo-loop mentioned above controls the frequency difference between the CPT-generating frequency components by adjusting the modulation frequency. Since the modulation frequency generator is held at a frequency determined by the low states of the absorber, the output of the modulation frequency generator provides a frequency standard having high precision provided the difference in energy of the corresponding low energy states of the quantum absorber remains constant.
As noted above, to be useful as a CPT-based frequency standard, the device must be insensitive to environmental conditions. Since the CPT is induced by the applied electromagnetic field at the frequencies corresponding to the transition frequencies from the low energy states to the common high energy state, the absorber often exhibits an AC Stark shift. As a result, the energy difference between the two low energy states will vary as a function of the intensity of the CPT-generating frequency components applied to the quantum absorber.
One method for reducing the AC Stark shift operates by introducing additional frequency components (AC-Stark-shift-manipulating frequency components) into the applied electromagnetic field. If the AC-Stark-shift-manipulating frequency components have the correct intensities and frequencies relative to the intensities of the CPT-generating frequency components discussed above, the AC Stark shift is substantially reduced. In this case, the difference in energy between the two low states will be insensitive to the intensities of the CPT-generating frequency components. If a modulated laser is used to generate the CPT-generating frequency components, the intensities of the AC-Stark-shift-manipulating frequency components are readily changed by adjusting the amplitude of the modulation signal applied to the laser. The frequencies of the AC-Stark-shift-manipulating frequency components are determined by the modulation frequency. In this example, both the CPT-generating frequency components and the AC-Stark-shift-manipulating frequency components are generated by modulating the same laser; the ratio of intensity of any one frequency component to any other frequency component is determined by the modulation. Therefore the AC Stark shift is insensitive to the total incidence intensity of the laser beam.
While the inclusion of the AC-Stark-shift-manipulating frequency components substantially corrects the problems introduced by the AC Stark shift, the AC-Stark-shift-manipulating frequency components reduce the signal-to-noise ratio in the output of the detector used to measure the intensity of electromagnetic radiation transmitted through the quantum absorber. Hence, these components reduce the effectiveness of the servo loop that corrects for variations in the frequency difference between the CPT-generating frequency components. The reduction in signal-to-noise ratio results from a difference in absorption between the CPT-generating frequency components and the AC-Stark-shift-manipulating frequency components. The AC-Stark-shift-manipulating frequency components suffer much less absorption in the quantum absorber than the two CPT-generating frequency components. Since the detector measures the sum of the powers of each of the frequency components in the electromagnetic field transmitted through the quantum absorber, the power in these AC-Stark-shift-manipulating frequency components forms a more or less constant background signal that is superimposed on the signal represented by the variation in the intensities of the two CPT-generating frequency components as the frequency difference between them is varied. This background signal reduces the signal-to-noise ratio.